1. Field of the Invention
The invention relates to synthetically generated displays and particularly to cathode ray tube displays utilizing digitally generated rasters.
2. Description of the Prior Art
Digital raster display generators are known in the prior art that utilize permanently wired specially designated circuits for generating the video signals during the time intervals defined by the digital circuitry generating the raster. Such systems generally utilize a unique permanently wired symbol generator for each raster symbol or pattern to be displayed. Such systems have the disadvantages that they are not programmable and that they require large amounts of permanently wired circuitry.
Another prior art digital display generator utilizes a full field refresh memory system where each resolution element of the display is defined by a group of memory bits in accordance with the shades of gray desired for the display. The picture is loaded into the memory from a computer and the entire memory is read out in synchronism with the digital circuitry generating the raster. The serial digital memory output words are converted to analog form and are transmitted to the display for each frame refresh. This prior art display system has the disadvantage that it requires an inordinately large memory for storing the digital words corresponding to all of the resolution elements of the frame. For displays of nominal size utilizing an adequate contrast range, memory capacities of between 500,000 and 1 million bits are required. Additionally, the time required to program the memory renders its use prohibitive with present day technology for rapidly changing display formats. It is also appreciated that because of the necessity for rapid readout of the large memory required, a high speed memory system would of necessity be utilized which system tends to be complex, expensive and critical in operation.
In addition to the above, the desirable ability to superimpose display symbols is generally difficult to achieve in the prior art.